This invention relates to two-piece pistons for internal combustion engines. Particularly, the invention relates to a piston of the articulated type intended primarily for diesel engines.
In the present days there is a growing tendency toward the use of articulated, or two-piece, pistons on diesel engines. In the articulated piston the head and the skirt are independent members: the skirt is assembled by means of a wrist pin on bosses located at the lower end of members depending from the head. With the piston fitted in the cylinder there is no contact between the head and the skirt portions. The basic feature of the articulated piston is that its components perform independent functions: the head, provided with the ring zone, bears the pressure of the combustion gases, while the skirt functions as a guide of the piston in the cylinder and supports the side loads transmitted by the inclination of the connecting rod.
The mounting of this type of piston in the cylinder can be made with small clearances due to the use of ferrous metals, which have a lower coefficient of thermal expansion, approximately that of the cylinder liner, which favors a better piston ring stability owing to a smaller lateral movement of the piston, thereby affording a lower lubricating oil consumption and a reduction of blow-by. Moreover, the absence of contact between the skirt and the head results in a reduced temperature on the skirt, thereby affording smaller mounting clearances and accordingly a decrease of the piston noise level.
Generally, the top portion, or head, of articulated pistons is made of a ferrous metal which is more resistant to the high thermal and mechanical loads, the skirt being generally of a cast or forged aluminum alloy. The use of a ferrous metal for the head affords placing the upper compression ring groove very close to the piston top, which would be virtually impracticable on conventional aluminum alloy pistons.
On high output diesel engines, the high temperatures on the piston top, especially on the ring zone and combustion chamber, make these areas extremely susceptible to problems. On the ring zone, very high temperatures may cause an excessive and early wear of the upper compression ring groove, degradation of the lubricating oil, build-up of carbon deposits and ring sticking. On the combustion chamber, these problems are represented by thermal cracks which tend to propagate and ultimately cause a breakage of the piston top. Therefore, it is necessary to provide a decrease of temperature of these regions in order to solve this problem.
One well-known approach to effect this cooling on articulated pistons (see U.S. Pat. No. 4,180,027 to Taylor) is to provide the upper portion of the skirt, next to the head lower portion, with a tray, and the head region, between the central portion and the ring zone, with a recess, in such a fashion that the tray and the recess define a semi-open chamber for circulation of the cooling oil which is supplied into the chamber by means of an injection nozzle. The reciprocating motion of the piston causes the cooling oil in the chamber to make a movement known as cocktail shaker, thereby removing part of the heat of the ring zone and combustion chamber portions, thus reducing the temperature of said regions. The heated oil exits the cooling chamber through one or more holes located at suitable points, at the same time that a quantity of fresh oil is fed into the chamber, thus defining a continuous circulation of coolant.
During the ascending and descending travel of the piston, inertia phenomena arising out of deceleration and acceleration forces cause a part of the oil injected into the chamber to be retained on the piston bottom,, i.e., on the lower portion of the combustion chamber. As soon as the action of such inertial forces ceases, a great portion of the cooling oil until then retained drops or flows into the tray on the top of the skirt. Together with this cooling oil the quantity of the oil fed by the injection nozzle also into the tray is ultimately higher than the tray capacity, causing part of the excess oil to overflow the inner wall of the tray into the crankcase. However, part of the excess oil overflows the outer wall of the tray and passes toward the cylinder through the opening between the head and the skirt. This excess oil, which is then trapped between the piston skirt and the cylinder, results in an overwork of the oil control ring which cannot prevent part of the oil from reaching the piston top land where it is then burned in the combustion process. This brings about a higher lubricating oil consumption, the build-up of carbon deposits on the top land and a higher level of emissions, especially of particulate matter.